I. Field of the Invention
The present invention relates generally to position location systems. More particularly, the present invention relates to systems and methods for determining the physical location of a mobile telephone within a cellular communication system.
II. Description of the Related Art
Recent developments in global positioning satellite (GPS) systems and terrestrial mobile communications make it desirable to integrate GPS functionality into a mobile communication device, such as a mobile telephone, in order to support various position location functions and features. A wireless link exists between a mobile communication device within a terrestrial mobile communications system and a base station within the communications system. A base station is typically a stationary communication device that receives wireless communications from, and transmits wireless communications to, a wireless mobile communication device. The base station also communicates with communication networks to complete the connection between the mobile communication device and an end-point device, such as another mobile communication device, a conventional telephone, a computer or any other such device. This wireless link may be used to communicate position location information between the mobile communication device and the base station in order to improve the performance of the GPS receiver within the mobile communication device. In particular, certain functions that must be performed in order to locate the position of a mobile communication device in accordance with a GPS system can be performed by the base station, rather than by the communication device. By "off-loading" some of the functions to the base station, the complexity of the communication device can be reduced. Furthermore, since the base station is stationary, the location of the base station can be used to assist in locating the position of the communication device.
Many services, such as CDMA Tiered Services (described in industry standard TR45.5.2.3/98.10.xx.xx, CDMA Tiered Services Stage 2 Description, Version 1.1, published by the Telecommunication Industry Association/Electronics Industry Association (TIA/EIA)), require a wireless telephone to be capable of determining its location while in an idle state. The location must then be displayed to the user. In the idle state, the wireless telephone monitors transmissions from a base station over a control channel broadcast by the base station. For example, in an industry standard IS-95 CDMA system (as defined by industry standard IS-95, published by the TIA/EIA), the base station transmits a paging channel. Each of the telephones capable of receiving signals from a particular base station will monitor information broadcast on the control channel to determine whether incoming calls or other data are intended for that telephone.
A GPS receiver typically measures the range to at least four GPS satellites. If the locations of the satellites and the ranges from the phone to the satellite are known at the time the measurement is made, then the location of the phone can be computed. Since GPS satellites orbit around the Earth, the relative position of the GPS satellites with respect to the earth changes with time. The location of a GPS satellite can be determined by having a description of the orbit of the satellite along with the time when the satellite position is to be computed. The orbits of GPS satellites are typically modeled as a modified elliptical orbit with correction terms to account for various perturbations.
In a GPS system, the orbit of the satellite can be represented using either an "Almanac" or an "Ephemeris". An Ephemeris provides data that represents a very accurate representation of the orbit of the satellite. An Almanac provides data that represents a truncated reduced precision set of the parameters provided by the Ephemeris. Almanac data is much less accurate than the detailed Ephemeris data. Almanac accuracy is a function of the amount of time that has elapsed since the transmission. Table 1 shows the relationship between the age of the information (i.e., amount of time which has elapsed since the information was transmitted) and the accuracy of the information.
TABLE 1 ______________________________________ Age of data time (from transmission) Almanac Accuracy (m) ______________________________________ 1 day 900 1 week 1200 2 weeks 3600 ______________________________________
In addition, the Almanac provides truncated clock correction parameters. The almanac time correction provides the time to within 2 .mu.sec of GPS time. However, the satellite location and clock correction computed using Almanac data are not useful to compute the location of the phone because of the low accuracy as shown in the above Table 1.
Certain methods for computing the location of a device require measuring the ranges to the satellites at the wireless phone, and then transmitting these ranges to a server connected to the base station. The base station uses these ranges, along with the locations of the satellites at the time the range measurements were made, to compute the location of the phone. This computed location may be displayed to the user or sent to any other entity that needs the location. This method is suitable for a phone that has a dedicated traffic channel. However, this method is not suitable for phones in the idle state, because the phone lacks a dedicated traffic channel over which to send the information to the base station during idle state.
In the absence of a dedicated traffic channel over which to communicate with the base station, the phone may use a shared access channel to send information to the base station. However, transmitting measured ranges to the base station over the shared access channel (which is commonly used to establish a call to or from the phone) can have a significant impact on the capacity of the shared access channel and on the life of the battery that powers the phone. Hence, it is not practical to transmit measured ranges to the base station. This requires that the phone be able to compute its own location. In order to do so, the phone must know the locations of the GPS satellites, and the errors in the GPS satellite clock (since an accurate GPS satellite clock is required to determine the range measurements accurately). This information must be transmitted to the phone over the control channel. However, even transmitting this information to the phone creates a significant burden on the control channel.
Under conventional conditions, the control channel has to carry very large amounts of information. The control channel has a limited capacity to carry messages. Hence, it is not possible to convey extensive GPS information over the control channel. Furthermore, the information must be transmitted in a form that allows the information to be used for a relatively long time after it has been received.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.